Yesterday, I finally installed mplayer on my good old Ultra5 with Mach64 on-board graphic card, and I noticed that I had no xv extention on my X server. After some research, I find a way (not the best I think ...) to install it quite quickly so I write this little howto.

Conclusion
There can be some other way to make xv work with the mach64, the most obvious is using the gatos drivers, but you will need to recompile the whole xfree to use them.
The other thing is that you should be able to make DRI for Mach64 with this tricks, but Mesa opengl seems to be broken on sparc (bug # 19776 )

In fact, the tricks works only to get X-Video support. For DRI, there is some problems to fix :
the library libgl is broken on sparc with xfree 4.3, but there is a workaround (just tried it yesterday) : you rebuild a working libgl using the DRI-CVS tree and this workaround:
https://bugs.gentoo.org/show_bug.cgi?id=19776#c8
there is a problem with the sparc.c : it tries to include a file that is missing (but I can't remember the name of the file), just try and comment the #include statement in sparc.c and it should work
then to get DRI working, I have a problem too, I can build the mach64.o kernel module, but I can't insmod it (pb of unresolved symbol).
I'm still trying and will post if I get results